There are many styles of writing, and I don't believe blogging on it's own helps a person to improve their mastery of language. Writing with conscious effort does. What I mean is that a lot of people just write as they think, and most don't seem to notice the errors in their work, in the same way that they don't notice that they are speaking poorly.
To really benefit, to really see an improvement, you also need to be aware that you can improve. You need to be aware that there is a benchmark above your current level that you want to reach.
Typically people are made aware of a need to improve when they start communicating with others who have a better grasp of the language. So, people who regularly read NetBuilders and get involved in forum discussions would notice the different styles of writing, and some might feel that they need to improve. This is probably why so many people in this thread think blogging has improved their writing, and why some feel their English has improved.
However, I don't believe blogging on its own is going to help, especially if the only other blogs you read are written by people with the same level of understanding as you. For example, many bloggers have a particular interest, such as gaming, sport, or mommy blogs, so they tend to comment on similar blogs, and in many cases a subculture language develops that isn't a great improvement.
Another example might be people from countries like India, the Philippines, Eastern Europe, who blog in English, but might actively seek out other people from their country because of shared cultural backgrounds. I doubt their writing would improve much if the other bloggers are also second language speakers.
So, those people whose writing improves through blogging are probably aware that their writing can improve, and are then making a conscious effort to improve; but the average blogger isn't aware they can improve, and don't.
